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The evolution of O i over 3.2 < z < 6.5 : reionization of the circumgalactic medium.

Becker, George D. and Pettini, Max and Rafelski, Marc and D’Odorico, Valentina and Boera, Elisa and Christensen, Lise and Cupani, Guido and Ellison, Sara L. and Farina, Emanuele Paolo and Fumagalli, Michele and López, Sebastian and Neeleman, Marcel and Ryan-Weber, Emma V. and Worseck, Gábor (2019) 'The evolution of O i over 3.2 < z < 6.5 : reionization of the circumgalactic medium.', Astrophysical journal., 883 (2). p. 163.


We present a survey for metal absorption systems traced by neutral oxygen over 3.2 < z < 6.5. Our survey uses Keck/ESI and VLT/X-Shooter spectra of 199 QSOs with redshifts up to 6.6. In total, we detect 74 O i absorbers, of which 57 are separated from the background QSO by more than 5000 km s−1. We use a maximum likelihood approach to fit the distribution of O i λ1302 equivalent widths in bins of redshift and from this determine the evolution in number density of absorbers with W 1302 > 0.05 Å, of which there are 49 nonproximate systems in our sample. We find that the number density does not monotonically increase with decreasing redshift, as would naively be expected from the buildup of metal-enriched circumgalactic gas with time. The number density over 4.9 < z < 5.7 is a factor of 1.7–4.1 lower (68% confidence) than that over 5.7 < z < 6.5, with a lower value at z < 5.7 favored with 99% confidence. This decrease suggests that the fraction of metals in a low-ionization phase is larger at z ~ 6 than at lower redshifts. Absorption from highly ionized metals traced by C iv is also weaker in higher-redshift O i systems, supporting this picture. The evolution of O i absorbers implies that metal-enriched circumgalactic gas at z ~ 6 is undergoing an ionization transition driven by a strengthening ultraviolet background. This in turn suggests that the reionization of the diffuse intergalactic medium may still be ongoing at or only recently ended by this epoch.

Item Type:Article
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Publisher statement:© 2019. The American Astronomical Society. All rights reserved.
Date accepted:26 August 2019
Date deposited:31 October 2019
Date of first online publication:01 October 2019
Date first made open access:31 October 2019

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